@astrokatie.com I am reading your book and I HAVE A QUESTION! 🤯 May I ask it here? (Maybe you answer it later in the book but it keeps poking at me as I read...)
Okay, so, spacetime. We're looking at a galaxy that's 13 billion light years away, so as you write, we're seeing what that galaxy looked like 13G years ago. But, 13G years ago, it was also much, much closer to us.
But the light between us has still been travelling for 13G years, starting when we were close together. Does that mean the speed of light has been speeding up as the universe expands—because it would have had to have been very slow for the light to have not reached us sooner when we were closer?
Or does it mean that distance/space has actually somehow remained constant even as it "stretches"? i.e. a light year now is the same as it was 13G years ago? But if so, then what does it even mean to say the universe is expanding?
Like, if the speed of light is constant, and distance is jus distance, then light emitted from a body 13G years ago should have reached us much sooner than 13G years, because it was much closer to us when it was omitted, right?
Well the space in between has been expanding in the meantime so we do see light that was emitted 13G years ago but the thing that emitted the light is MUCH farther away now than it “looks” in the image we see
So a galaxy we see 13 billion years in the past is actually much farther away than 13 billion light years now. Something like 40-something billion light years, perhaps
So I’m pretty careful when talking about the most distant galaxies not to ever claim that something whose light reached us from 13 billion years ago is 13 billion light years away, because it isn’t. For 1 billion light years or less it’s close to correspondence but not for the really distant stuff
Thank you for this! Reminds me of what a friend once said, pondering what other life forms see when they look at Earth from those distances & why they haven't visited. Joking, he said they see dinosaurs or no measurable life so they're not interested.
until the sphere of humanity’s electromagnetic traces finally reaches an aeons-dormant listening buoy in deep space, alerting the masters of the dark forest that they might be interested in a damp little world swirling around a middle-aged yellow dwarf…
Thank you! This raised another question for me, but then I think I might have answered it myself, so I think I'm gonna go read more and then come back maybe 😂
Besides "far away things are getting away from us faster", do we have other evidence that space is expanding? Could there be some other (maybe otherwise nonsensical) theory that explains that phenomenon, like… "old light gets red-shifted because x"? Is expansion the only explanation that fits?
So there have been other ideas proposed, such as the disproven "tired light" hypothesis (which you can look up) but the fact that we also see cosmological time dilation -- distant supernovae seem to explode more slowly, e.g. -- very much supports the expansion idea
Ok stupid question. Does Motion within those galaxies appear to be moving in relative SLO motion from our point of view. Due to the stretching? And the light taking longer to get here?
Sorry if the question doesn’t make sense.
Yeah, in principle, though there are only a few things we can see that happening with. Supernova definitely look slowed down though. bsky.app/profile/astr...
Yes! And when things are moving away from us with expansion (redshifted) we see them in slow-motion! When we watch really distant supernovae we seem them actually take longer! 🤯
